Rhodospirillum rubrum is a facultatively phototrophic purple nonsulfur bacterium. Under reduced oxygen concentration, this organism forms an intracytoplasmic membrane (ICM) that is the site of the photosynthetic apparatus (Collins, M. L. P., and C. C. Remsen, The purple phototrophic bacteria, p. 49-77, In J. F. Stolz (ed.), Structure of Phototrophic Procaryotes. CRC Press, Boca Raton Fla., 1990; Crook, S. M., et al., J. Bacteriol. 167:89-95, 1986; Hessner, M. J., et al., J. Bacteriol. 173:5712-5722, 1991). This apparatus consists of the light-harvesting antenna (LH) and the photochemical reaction center (RC). The pigment-binding proteins, the LH xcex1 and xcex2 and the RC-L and -M, are encoded by the puf operon while RC-H is encoded by puhA. The nucleotide sequences of puhA and the puf operon have been determined in R. rubrum (Bxc3xa9langer, G., et al., J. Biol. Chem. 263:7632-7638, 1988; Bxc3xa9rard, J., et al., J. Biol. Chem. 261:82-87, 1986; Bxc3xa9rard, J., and G. Gingras, Biochem. Cell Biol. 69:122-131, 1991) and related bacteria (Donohue, T. J., et al., J. Bacteriol. 168:953-961, 1986; Kiley, P. J., et al., J. Bacteriol. 169:742-750, 1987; Michel, H., et al., EMBO J. 5:1149-1158, 1986; Michel, H., et al., EMBO J. 4:1667-1672, 1985; Weissner, C., et al., J. Bacteriol. 172:2877-2887, 1990; Williams, J. C., et al., Proc. Natl. Acad. Sci. 81:7303-7307, 1984; Williams, J. C., et al., Proc. Natl. Acad. Sci. 80:6505-6509, 1983; Youvan, D. C., et al., Proc. Natl. Acad. Sci. 81:189-192, 1984; Youvan, D. C., et al., Cell 37:949-957, 1984).
R. rubrum may grow phototrophically under anaerobic light conditions or by respiration under aerobic or anaerobic conditions in the dark. Because R. rubrum is capable of growth under conditions for which the photosynthetic apparatus is not required, and because the photosynthetic apparatus and the ICM may be induced by laboratory manipulation of oxygen concentration, this is an excellent organism in which to study membrane formation (Collins, M. L. P., and C. C. Remsen, supra, 1990; Crook, S. M., et al., supra, 1986).
In previous studies, the puf region was cloned and interposon mutations within this region were constructed (Hessner, M. J., et al., supra, 1991). R. rubrum P5, in which most of the puf genes were deleted, was shown to be incapable of phototrophic growth and ICM formation. P5 was restored to phototrophic growth and ICM formation by complementation with puf in trans (Hessner, M. J., et al., supra, 1991; Lee, I. Y., and M. L. P. Collins, Curr. Microbiol. 27:85-90, 1993). These results imply that in R. rubrum the puf gene products are required for ICM formation. These results differ from those obtained with a puf interposon mutant of Rhodobacter sphaeroides (Davis, J., et al., J. Bacteriol. 170:320-329, 1988) which was phototrophically incompetent but was still capable of ICM formation (Kiley, P. J., and S. Kaplan, Microbiol. Rev. 52:50-69, 1988). In the case of R. sphaeroides, the formation of ICM in the absence of the puf products may be attributable to the presence of an accessory light-harvesting component (LHII) encoded by puc (Hunter, C. N., et al., Biochem. 27:3459-3467, 1988). This implies that R. rubrum is a simpler model for studies of membrane formation.
Because the puf-encoded proteins are required for ICM formation in R. rubrum and because the RC is assembled from puf and puhA products, it is important to evaluate the role of puhA-encoded RC-H in RC assembly and ICM formation in R. rubrum. 
Cheng, et al., J. Bacteriol. 182(5):1200-1207, 2000 and Yongjian S. Cheng, xe2x80x9cMolecular Analysis of Biochemical Intracytoplasmic Membrane Proteins,xe2x80x9d PhD thesis, UW-Milwaukee, August, 1998 describe the cloning, mutation, and complementation of the puhA region of R. rubrum. (Both of these documents are incorporated herein by reference.) The present application proposes a model for the preparation of proteins, preferably membrane proteins.
In one embodiment, the present invention is a method of expressing protein comprising the steps of placing a DNA sequence encoding a protein or peptide in an expression vector that contains a regulatable promoter expressible in Rhodospirillum rubrum and expressing the protein within a bacterial host, wherein the host has extra capacity for membrane formation and wherein the host is a member of the genus Rhodospirillum.
In a preferred embodiment of the present invention, the protein or peptide is a membrane protein or peptide and/or the protein or peptide is a heterologous protein or peptide.
In another preferred form of the present invention, the host is Rhodospirillum rubrum. 
In another embodiment, the present invention is a protein expression system. In one embodiment, the protein expression system encompasses a vector comprising a DNA molecule encoding the protein or peptide in an expression vector containing a regulatable promoter expressible in R. rubrum. The vector is contained within a host, preferably R. rubrum with extra capacity for membrane formation.